As portable electronic equipment spreads rapidly, the specification of the batteries used for them have been becoming more strict year after year. Especially, batteries with a compact and slim size, high-capacity, excellent cycle characteristics, and stable performance are required. In addition, in the field of secondary batteries, lithium nonaqueous electrolyte secondary batteries with higher energy density than other batteries have been drawing attention, and the market share of lithium nonaqueous electrolyte secondary batteries in the secondary battery market has shown a large increase.
FIG. 1 is a perspective view showing a longitudinal section of a commonly manufactured cylindrical-shaped nonaqueous electrolyte secondary battery. The nonaqueous electrolyte secondary battery 10 uses a rolled electrode 14 formed by rolling a positive electrode plate 11 and a negative electrode plate 12 with a separator 13 interposed therebetween, and is manufactured by the following steps: insulting plates 15 and 16 are placed on upper and lower faces of the rolled electrode 14, respectively; then the whole is put into a cylindrical-shaped battery outer can 17 also serving as a negative electrode terminal; a negative electrode current collecting tab 12a connected to the negative electrode plate 12 is welded on an inner bottom part of the battery outer can 17 and also a positive electrode current collecting tab 11a connected to the positive electrode plate 11 is welded on a bottom plate part of a current breaking cover 18 equipping a safety apparatus; a predetermined nonaqueous electrolyte is poured from an opening of the battery outer can 17; and then the battery outer can 17 is sealed up with the current breaking cover 18. Such a nonaqueous electrolyte secondary battery shows excellent effects of higher battery performance and reliability.
In the nonaqueous electrolyte secondary batteries, as a positive electrode active material, one of or a mixture of a plurality of lithium transition-metal composite oxides represented by LixMO2 (where M is at least one of Co, Ni, and Mn) capable of absorption and desorption of lithium ions reversibly, that is, LiCoO2, LiNiO2, LiNiyCo1-yO2 (y=0.01 to 0.99), LiMnO2, LiMn2O4, LiNixCoyMnzO2 (x+y+z=1), LiFePO4, or the like, is used.
Among them, lithium cobalt oxide (LiCoO2) is mainly used because its various battery characteristics are especially higher than those of other oxides. However, since cobalt is expensive and the amount of cobalt in natural resources is small, in order to use lithium cobalt oxide as the positive electrode active material of the nonaqueous electrolyte secondary batteries continuously, it is desired that the nonaqueous electrolyte secondary batteries have higher-performance and longer-life-time.
For example, Japanese Patent No. 3244314 discloses an invention of a nonaqueous electrolyte secondary battery using a lithium transition-metal composite oxide represented by LiaMbNicCodOe (where M is at least one metal selected from the group consisting of Al, Mn, Sn, In, Fe, Cu, Mg, Ti, Zn, and Mo, and ranges are 0<a<1.3, 0.02≦b≦0.5, 0.02≦d/c+d≦0.9, and 1.8≦e≦2.2, and further b+c+d=1 and 0.34<c) (for example, Li1.0Mn0.1Ni0.45Co0.45O2.0) as the positive electrode active material. Since a crystal structure of the positive electrode active material is comparatively stable even if Li is extracted at the time of charging, even if charging and discharging are performed repeatedly, the crystal structure is not destroyed and reversible charging and discharging can be performed.
When considering to increase the capacity of the nonaqueous electrolyte secondary batteries using lithium cobalt oxide as the positive electrode active material, achievement of higher-performance and higher-safety of the batteries is a fundamental problem. As discussed above, so-called three-component positive electrode active materials containing three components of Ni, Co, and Mn with various compositions in the structures are known. However, the conventional three-component positive electrode active materials have advantages and disadvantages, respectively, for example, a high Mn composition improves the thermal stability but is apt to deteriorate the capacity, and a high Ni composition improves the capacity but is apt to deteriorate the safety. Accordingly, in order to accept higher-performance and higher-safety of the three-component positive electrode active material containing three components of Ni, Co, and Mn in the structure in future, it is needed to raise the level of the whole performance and to cover the disadvantages.